1. Field of the Invention
The present invention relates generally to the controlled growth of nanosized metal particles and more specifically to excess enthalpy resulting from the controlled growth of nanosized metal particles in the presence of hydrogen or deuterium.
2. Description of the Prior Art
The study of dispersed metals has a long history because of their use as catalysts. It is well-known in the art that as the size of the metal particles decrease, the activity increases to a point. For the purposes of this application, papers concerning the ratio of hydrogen to palladium atoms and the heats of adsorption of hydrogen into palladium are referenced, although the rate of organic chemical bond-forming and cracking reactions also vary with particle size. Many papers reference hydrogen reactions only, but for the purpose of this application, hydrogen and deuterium are considered identical in chemical nature for the ratios of uptake with a metal catalyst. For example, Chou, et al., and Huang, et al., have shown that the uptake of hydrogen increases rapidly when the particle size of the dispersed palladium in an oxide matrix decreases to less 1 nm or less. (Shu-Chin Chou, et al., “Isosteric Heat of Sorption of Dihydrogen on Alumina-supported Palladium,” J. Chem. Soc. Faraday Trans., 91, 949-51 (1995); Sheng-Yang Huang et al., “Chemical Activity of Palladium Clusters: Sorption of Hydrogen,” J. Phys. Chem. B, 110, 21783-87 (2006)). Although, they did not state the particle size specifically, data from Huang can be used to estimate the particle size and approximate ratio of H:Pd as shown in Table 1. The heat of adsorption also increases with decreasing particle size. Chou, et al., studied a number of supports and preparation conditions and also showed that the heat of adsorption and loading ratio increased with decreasing particle size. (Pen Chou, et al., “Calorimetric Heat of Adsorption Measurements on Palladium I. Influence of Crystallite Size and Support on Hydrogen Adsorption,” J. of Catalysis, 104, 1-16 (1987)). However, the estimated particle size in Chou's work was greater than 1.6 nm. Aben showed that hydrogen absorption could be used to estimate particle size and that the H:Pd ratio also increased with decreasing particle size, reaching a maximum H:Pd ratio of 0.83 in his study using ion exchanged silica. (P. C. Aben, “Palladium areas in supported catalysts: Determination of palladium surface areas in supported catalysts by means of hydrogen chemisorption,” Journal of Catalysis, 10, 224-29 (1968)). The smallest size that Aben measured was 2.5 nm, and results showed that high pretreatment temperatures increased particle growth.
TABLE 1Estimated particle sizes and H/Pd ratios as calculated fromHuang. Note the sensitive dependence on the loading ratiowith particle size. The more chemically accessible particles(>5 nm) show a loading similar to bulk palladium of 0.6.Heat ofEstimatedHydrogenParticleAdsorptionRatio H:PdPreparationSize (nm)(kJ/mole)@ 0.2 barPd Powder9940.551.86% Pd/SiO2 (IW)~4920.6810% Pd/SiO2 (SG)1.11310.95% Pd/SiO2 (SG)11831.05
As the particle size must be small for high H:Pd ratios, one must disperse the particles on a support to keep them from sintering and growing too large. P. A. Sermon stated that even heating palladium black to 98° C. would cause sintering of the particles. (P. A. Sermon, “Characterization of palladium blacks: I. A novel hydrogen pretreatment and surface area determination of palladium,” J. of Catalysis, 24, 460-66 (1972)).
As is well known in the art (for example, see Huang and references cited therein), there are three general methods to prepare dispersed metals on supports: (1) incipient wetness impregnation, where a solution of metal precursor is absorbed on the support. The amount of solution is just enough to wet the support. A variation on this method is wet impregnation where the amount of solution is greater than needed to just wet the support and the excess is removed at low temperature. (2) Ion exchange, where ions associated with the support are replaced with the metal ion of interest, generally using aqueous solutions of the metal of interest. Or (3) Sol-gel, where solutions of the metal are suspended in a growing polymer, which is generally inorganic in nature. After forming the supported metal precursor, the support is generally dried and calcined to remove water and organics. The heating may be done in air or an inert gas or in the presence of a reducing agent such as hydrogen. Finally, the metal ions are reduced to metal nanoparticles with a reducing agent such as hydrogen. This may be done at elevated temperatures. Elevated temperatures and high metal loading appear to increase particle size above 2 nm size and should be avoided during the preparation of the supported metal particles used in the present invention.
It is well-known in the art that repeated cycling of a supported catalyst can cause sintering and particle growth. This problem may be reduced by encapsulating the metal particles in a matrix such as a zeolite, a sol-gel, or a protective polymer. (D. G. Narehooda et al., “X-ray diffraction and H-storage in ultra-small palladium particles,” International Journal of Hydrogen Energy, 34, 952-60 (2009)). Also, the use of zeolites as supports for metal particles is well-known in the art. For example, see: K. P. Prasanth et al., “Hydrogen uptake in palladium and ruthenium exchanged zeolite X,” Journal of Alloys and Compounds, 466, 439-46 (2008); Kh. M. Minachev et al., “Deuterium Exchange with the Surface of Zeolite Catalysts 5. Palladium-Containing Zeolites,” Academy of Sciences of the USSR, Moscow, translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2678-82 (December, 1978); and J. Michalik et al., “Studies of the Interaction of Pd3+ and Pd+ with Organic Adsorbates, Water, and Molecular Oxygen in Pd—Ca—X Zeolite by Electron Spin Resonance and Electron Spin-Echo Modulation Spectroscopy,” J. Phys. Chem., 89, 4553-60 (1985).